IX. THE ANALYSIS AND VALUATION OF MANUEES 175 



gravity 1*09 at 20 and contains about 370 grammes of crystallised 

 citric acid, neutralised with ammonia, in two litres of water (American) , 

 or 333 grammes citric acid (Petermann), or about 360 grammes (Halle 

 process) in two litres. 



The temperature and time of digestion recommended by different 

 workers vary : 30 or 40 for half an hour (Frankland) ; 50 for half an 

 hour (Halle station) ; 35 to 40 for one hour (Norway stations) ; 65 

 for half an hour (American official chemists). 



In any case, the liquid is filtered and washed, and the phosphoric 

 acid in the residue determined as usual. 



Thus, direct determinations are made of 



(1) Phosphoric acid soluble in water. 



(2) Total phosphoric acid. 



(3) Phosphoric acid insoluble in ammonium citrate. 



The difference between (2) and (3) gives the citrate and water- 

 soluble phosphoric acid, and by subtracting (1) the citrate- soluble or 

 "reverted" phosphoric acid is obtained. 



In the case of basic slag, the tetracalcium phosphate is soluble in 

 citric solution, but as there is always more or less free lime present, 

 Wagner recommends that sufficient citric acid be added to neutralise 

 the free lime in 5 grammes of the slag, and then 200 cc. of acid 

 ammonium citrate (made by dissolving 160 grammes of citric acid in 

 water, adding 27*93 grammes of real NH 3 and diluting to 1 litre). 

 After filtering, the phosphorus pentoxide in solution is estimated by 

 separation with ammonium molybdate solution and precipitation by 

 magnesia mixture as usual. 



Potassium in presence of sulphuric acid, iron, lime and magnesia. 

 Usually the above bases and the sulphuric acid have first to be re- 

 moved by means of ammonia, ammonium carbonate or oxalate, and 

 barium chloride, necessitating the subsequent removal of the ammonium 

 compounds and excess of barium. By direct addition of platinum 

 chloride to the hydrochloric acid solution of the original substance, 

 evaporation to dryness, and washing, first with a little platinum chloride 

 solution, then with alcohol, next with a 20 per cent solution of ammon- 

 ium chloride, to which sufficient solid K 2 PtCl 6 has been added to saturate 

 it, and lastly again with alcohol, a pure double chloride of potassium 

 and platinum may be obtained. 



THE VALUATION OF MANURES FROM ANALYSIS. From 

 the percentage amount of the valuable constituent in a single manure 

 and its commercial value per ton, it is easy to calculate the cost of the 

 actual valuable ingredient per lb., or, as is perhaps more usual in this 

 country, " per unit," i.e., the value per ton of each per cent. 



For example, take nitrate of soda, containing, say, 15 '75 per cent 

 nitrogen, and assume its price per ton to be 8. 



Nitrogen, in this form, costs 



